Internal combustion engines are well known and widely used for propulsive power, electrical power generation, gas compression, liquid and gas transfer, and in various industrial applications. In a conventional four-cycle or two-cycle operating scheme, fuel and air is combusted within an engine cylinder to produce a rapid rise in pressure and induce linear travel of a piston coupled with a rotatable crankshaft. Spark-ignited engines typically employ a liquid petroleum distillate fuel such as gasoline, or various gaseous fuels in the nature of natural gas, methane, propane, and various mixtures such as biogas, landfill gas, and mine gas. Compression-ignition engines typically utilize fuels such as diesel distillate fuel, biodiesel, and still others. In recent years there has been significant interest in development of engines and operating strategies that are flexible with regard to fuel utilization. Fuel prices can be fairly dynamic and, moreover, certain fuels that have realized relatively increased abundance in recent years, such as natural gas, can have desirable combustion or emissions properties which are sought to be exploited. One type of engine design that allows for operation with different fuel types combines both a diesel distillate fuel and natural gas. Diesel alone is relatively easy to compression ignite, but can produce undesired emissions. Where natural gas is used as a fuel in a conventional compression-ignition engine, without modification the mixture of natural gas and air can fail to ignite, knock, or have combustion stability problems. Various strategies have been developed that predominantly burn natural gas while using a relatively smaller amount of diesel as a so-called pilot fuel. The diesel pilot fuel can ignite to in turn ignite the natural gas, offering relative predictability and reliability in the timing and manner of ignition and otherwise combining certain advantages of both fuel types. One example of such an engine is known from U.S. Pat. No. 6,032,617 to Willi et al.
The term “substitution ratio” is commonly used to describe the relative contributions of diesel fuel and gaseous fuel in a dual fuel engine at any one time, and can be understood generally as the extent to which gaseous fuel is substituted for what would otherwise be diesel fuel during single fuel engine operation. In certain dual fuel engines, particularly at relatively low engine load operating states, controlling the delivery of a relatively small amount of gaseous fuel can be difficult due at least in part to hardware and controllability limitations, or ignitability and/or other problems can be observed. Standard practice often includes disabling gaseous fuel delivery at low loads or other operating conditions where gaseous fuel utilization can be challenging.